The v-raf is the oncogene transduced by the 3611 MSV which causes fibrosarcoma in newborn mice. Using this oncogene as a probe, we have successfully characterized several raf-related celular counterparts, in man and mice, which exhibit significant nucleotide and amino acid homology so that they can be classified under a family of oncogenes. To elucidate the role of raf oncogenes in malignancy, we investigated the possibility of rearrangements of genomic raf DNA from various tumors and transformed cell lines. The DNA from a human breast carcinoma cell showed altered restriction patterns as compared to normal DNA when probed with c-raf-1 cDNA. Based on these initial results, we constructed a lambda genomic library and screened the library with the same probe. Two overlapping positive clones were further analyzed to map the rearranged locus. These studies demonstrated that two closely spaced EconR1 sites (approximately 200 bases apart) and an adjacent Sal-1 site, all clustered around 19.5 map unit, have been eliminated from the genome. These observations agree with the sizes of DNA fragments from the tumor cell DNA on a Southern blot analysis, where the 10.3 kb EcoR1 fragment is missing. In addition, the loss of a Sal-1 site at 19.5 map units would result in a larger Sal-1 fragment which we have confirmed by isolating a clone containing DNA from 4.4 through 22.4 map units which has no Sal-1 site. Further analysis revealed that the sequences representing exon 7 have been deleted from the genome. Thus, in these cells, the c-raf-1 oncogene is rearranged within the 5' half of the gene resulting in the deletion of an exon. Truncations within the NH2 terminal half of the raf protein generally characterize transforming versions of this gene.